Conventional drill bits include drag-type drill bits with polycrystalline diamond compact (PDC) inserts and cutters, which use a shearing action to break the material of the formation. Such PDC cutters and inserts are suitable for continuous loading, but are subject to failure under vibration, chatter, and impact forces.
Accordingly, torsional vibration is a significant concern when drilling using PDC bits. One prevalent cause of torsional vibration is a phenomenon known as “stick-slip.” Stick-Slip occurs when the rate of bit rotation suddenly slows or stops, such as when a hard interface or stringer is encountered by the bit. Although the drill bit momentarily slows or stops rotating, the drill string above continues to rotate, causing the drill string to wind up and torsionally store potential energy. This is the “stick” phase of the stick-slip cycle.
As the drill string winds up, drill string torque increases and drill string length decreases. At some point, either due to the increased drill string torque or decreased drill string length causing the bit to take a lesser bite of the formation, the formation shears and the bit rotates again. Due to the high energy stored in the drill string, the drill bit breaks free violently and over speeds as the drill string unwinds. This is the “slip” phase of the stick-slip cycle.
Stick-slip causes chatter, repeated impacts of the PDC cutters against the rock face, and drill string oscillation. Stick-slip is typically addressed by either increasing the bit speed or reducing the weight on bit. It is desirable, however, to provide a tool that mitigates torsional vibration without the need to increase the bit speed or reduce the weight on bit.